The present invention relates to a separation process for the one-step production of dimethyl ether (DME) from synthesis gas (syngas), a mixture of hydrogen (H2) and carbon monoxide (CO). In the one-step syngas-to-DME process, syngas is converted in a single reactor (DME reactor) to methanol and DME over a catalyst system with methanol synthesis (2H2+COxe2x86x92CH3OH), methanol dehydration (2CH3OHxe2x86x92DME+H2O), and water gas shift (H2O+COxe2x86x92CO2+H2) activities. Due to a chemical synergy among these three reactions, the single pass syngas conversion in the DME reactor, or productivity, is significantly greater than that in a methanol synthesis reactor, wherein the methanol synthesis reaction primarily takes place. Since reactors for syngas conversion are expensive equipment for high-pressure operation at elevated temperatures, greater conversion or productivity means smaller DME reactors, associated equipment, and operation. This can reduce the cost in the syngas conversion part of the process, and possibly lead to a more economic process for DME production than the traditional two-step process, namely, methanol synthesis followed by methanol dehydration in two separate reactors.
However, the downstream separation for the one-step process could be costly because of the high volatility of two reaction products, DME and CO2. CO2 is especially a problem. In general, there are three ways to deal with the CO2 problem. First, scrub out DME and methanol from the reactor effluent and let CO2 remain in the unconverted syngas and build up in the DME reactor loop. CO2 formation in the reactor will be suppressed when the CO2 concentration in the reactor loop reaches a certain level. In this approach, the product stream entering the downstream separation process is CO2 free, therefore it can be conducted at reasonable cost. However, this approach adversely affects the productivity of the DME reactor. Unless the H2:CO ratio in the syngas feed to the DME reactor, is very high (e.g.,  greater than 5, as in the WO Patent 96/23755 shown below), the final equilibrium CO2 concentration in the DME reactor loop will be large. The presence of a large amount of CO2 dilutes the reactants and hampers the synergy for the reaction system, resulting in a large decrease in the reactor productivity. One can avoid the build-up of a large amount CO2 by operating in very H2-rich (e.g., H2:CO greater than 5) regime. However, the productivity of the DME reactor in this regime is much lower than that when the DME reactor is operated in the optimal regime (H2:CO around 1). It could even be lower than the productivity of a syngas-to-methanol reactor at its best-feed composition (H2:CO=2) on a methanol equivalent basis. In brief, this suppressing-CO2-formation approach can minimize the CO2 handling cost; but it also takes away the very advantage of the one-step syngas-to-DME processxe2x80x94its high reactor productivity.
The second approach is to remove CO2 from the unconverted syngas in the above approach before it is recycled to the DME reactor, thereby preserving the high productivity of the DME reactor. A commercially available CO2 separation technology (physical or chemical absorption) can be used. However, since this requires an independent CO2 separation system, the cost could be so high that it may negate all the cost saving from the high reactor productivity. Furthermore, in the natural gas-based syngas-to-DME process, CO2 needs to be recycled to the syngas generation equipment to maintain a desired hydrogen:CO ratio. Since the pressure of the recovered CO2 from these absorption units is low, the compressing cost for returning CO2 to high-pressure syngas generation units will be high.
The third way to deal with the CO2 problem is to make it an integral part of the downstream product separation process. CO2 is removed, along with DME and methanol, from the reactor effluent. This would maintain the high productivity of the DME reactor, the very source of cost saving against the two-step DME process. The cost of the downstream separation will be higher than that of the two-step DME process due to the presence of CO2. However, one may be able to develop optimized separation schemes so that the CO2-induced cost will be much smaller than the cost saving by the high reactor productivity, therefore, warranting an economic one-step DME process. The objective of the current invention is to develop such an optimized separation scheme.
A number of separation schemes have been disclosed in the prior art for the one-step syngas-to-DME process. WO Patent 96/23755 and its equivalent U.S. Pat. No. 5,908,963 choose to avoid the CO2 problem by operating a fixed bed syngas-to-DME reactor in a H2-rich regime (H2:CO greater than 5). The reactor effluent is cooled in a condenser. The condensed reaction products, methanol, water and dissolved DME, are sent to two distillation columns for DME-methanol/water separation and methanol-water separation, respectively. Part of the gaseous stream from the condenser, containing unconverted syngas, DME and a small amount of CO2, is recycled back to the DME reactor; the rest is sent to a scrubbing column to recover DME. Methanol, from the water-methanol column, is used as the scrubbing solvent. The DME-methanol mixture from the scrubbing column is fed to a methanol dehydration reactor. Due to the high H2:CO ratio in the reactor feed, CO2 formation is suppressed with a small amount of CO2(e.g., 3 mol. %) in the reactor loop. However, the reactor is operated in a regime far away from the optimal conditions.
Methanol is also used as the scrubbing solvent in separation scheme disclosed in a paper by Bhatt, Toseland, Peng and Heydorn, 17th International Pittsburgh Coal Conference, Pittsburgh (September, 2000), for a 10 tons/day one step syngas-to-DME pilot plant (referred to as Bhatt""s paper hereafter). In this separation scheme, the effluent from a slurry phase syngas-to-DME reactor is first cooled to condense methanol and water out. The rest of the effluent is fed to a scrubbing column which uses methanol as a solvent. All DME, methanol and CO2 are removed from the unconverted syngas in the scrubbing column. The bottom stream from the scrubber is sent to a distillation column to regenerate methanol from DME and CO2. Due to the trial nature of the work, the DME and CO2 mixture was sent to flare without further separation.
A paper by Xie and Niu (Tianranqi Huagong, 24 (1999) p.28) examines different scrubbing solvents for DME separation, including methanol, water and methanol/water mixture. Methanol and 50/50 methanol/water mixture exhibited similar solubility to DME; both are better than pure water.
Chinese patent application No.1085824A to Guangyu et al. describes a downstream separation scheme for a one-step syngas-to-DME process. The water and methanol in the effluent from a fixed bed syngas-to-DME reactor are removed through a condenser and an absorption column, respectively. The rest of the reactor effluent enters into an extraction column. The unconverted syngas leaves the column from the top and is recycled to the DME reactor. A solvent is used in the extraction column to remove DME from the recycle stream. Water and ethanol are two solvents taught in the patent. When water is used as the extraction solvent, 5% of the CO2 in the effluent gas is also dissolved in the water. The water solution is sent to a stripping-distillation column to recover product DME and regenerate water. When ethanol is used as the solvent, considerable amount of CO2 (40%) is dissolved in the ethanol along with DME. The CO2 from the bottom of the extraction column is first removed by some method (not specified). The rest is sent to a stripping-distillation column for DME-ethanol separation.
A downstream CO2 separation scheme for a one-step syngas-to-DME process is described in a paper by Ohno, Ogawa, Shikada, Inoue, Ohyma, Yao and Kamijo, International DME Workshop, Japan (Sept. 7, 2000), (referred to hereafter as Ohno""s paper). The DME reactor effluent is chilled to remove DME, CO2 and methanol from the unconverted syngas, which is recycled to the DME reactor. The CO2 in the condensed liquid is removed in a CO2 column. The rest of the liquid is separated in a second column into product DME and methanol. The scheme also includes an amine-based absorption column to remove CO2 from the syngas generated by an autothermal reformer before the syngas is fed to the DME reactor.
U.S. Pat. No. 6,147,125 to Shikada et al. discloses a downstream separation scheme for a one-step syngas-to-DME process. The methanol and water in the DME reactor effluent is condensed out first. The rest of the effluent is fed to a scrubbing column to remove DME and CO2 from the unconverted syngas, which is recycled to the DME reactor. DME is used as the scrubbing fluid. The bottom of the scrubbing column is fed into a distillation column to separate CO2 from DME.
With the exception of the schemes disclosed in WO patent application 96/23755 and the Ohno""s paper, the other downstream separation schemes contain three common elements. First, a condenser is used to remove methanol and water from the reactor effluent. Second, a scrubber is used to remove DME and CO2 from the unconverted syngas, which is recycled to the DME reactor. The third element includes two or three distillation columns for recovery of the scrubbing fluid and separation of CO2, DME and, in some cases, methanol.
Scrubbing the volatiles (DME and CO2) from unconverted syngas is a key step in the downstream separation scheme summarized above. Since the solubility of DME in all known solvents is greater than that of CO2, the latter becomes the determining factor for the scrubbing operation, e.g., the amount of a solvent needed, the size of the scrubbing column. Therefore, it is the solubility of CO2 in the scrubbing solvent that has a strong impact on the economics of the process. High solubility means smaller amount of scrubbing solvent, smaller size of the scrubbing column and other downstream columns that involve the scrubbing solvent. If cooling or chilling is needed to prepare the scrubbing solvent, high CO2 solubility also means savings in the refrigeration cost.
The vapor pressure of the solvent also has significant impact on the economics. It is an important factor in determining the pressure and energy consumption of the downstream separation.
Water, methanol and DME have been used as the scrubbing solvent in the prior art, as described above. All three solvents have good solubility for DME but their solubility toward CO2 varies considerably. Water is the poorest solvent for CO2. When it is used as the scrubbing fluid, as in Chinese patent application No. 1085824A, most of the CO2 remains in the unconverted syngas. This CO2 needs to be either removed using an independent separation unit (e.g., physical or chemical absorption) or recycled to the DME reactor along with the unconverted syngas. This would either add additional separation cost or decrease the productivity of the DME reactor.
Methanol has very good solubility for DME. It is the scrubbing solvent for CO2 removal in the commercial Rectisol process. It is also used in the process described in Bhatt""s paper. However, CO2 solubility in methanol is not optimum. A large amount of methanol must be chilled to xe2x88x9230 to xe2x88x9250 F. to scrub CO2 from syngas effectively. This means a large capital investment for the scrubber and downstream distillation columns as well as a high operating cost for refrigeration.
DME has very good CO2 solubility, translating into smaller scrubber and downstream separation columns. However, it has several shortcomings. First, because of the high vapor pressure of DME, the unconverted syngas stream leaving the scrubbing column will contain a significant amount of DME. This DME will act as a diluent in the DME reactor feed, decreasing its productivity. Second, the high volatility of DME requires the downstream separation process to be operated at high pressures for given column condenser temperatures. Third, the process built around pure DME as the scrubbing solvent does not allow any methanol, one of the products of the DME reactor, to enter the scrubbing column. Otherwise, the methanol needs to be recovered at the bottom of a distillation column. This means one needs to use a large amount of energy to evaporate all DME, the product part as well as the solvent part. To condense the solvent part back to liquid will also consume a lot of energy.
In summary, there are two important cost issues associated with the scrubbing solvent. The first issue is that high solubility toward CO2 is desirable. Better solubility translates into a smaller scrubbing column, smaller downstream distillation columns and lower refrigeration duty. The second issue is the vapor pressure of the scrubbing fluid. Lower volatility means less negative impact on the DME reactor productivity and lower operating pressure for the downstream product separation section. Therefore, the ideal scrubbing solvent should have high solubility for CO2 and low volatility. None of the solvents in the prior art possess both of these properties. DME is good at dissolving CO2 but has high volatility. Methanol is less volatile but its solubility for CO2 is much to be desired. The current invention provides a means for addressing the limitations of using either DME or methanol as the scrubbing solvent.
In a general aspect, the process of the invention is an improvement in a process for production of dimethyl etherxe2x80x94in particular, for a process comprising the catalytic conversion of a synthesis gas in a dimethyl ether reactor, where the synthesis gas comprises a mixture of hydrogen and carbon monoxide, and the conversion results in an effluent mixture comprising dimethyl ether, methanol, carbon dioxide, water and unconverted synthesis gas, and wherein the effluent mixture is further processed to obtain a vapor mixture comprising dimethyl ether, carbon dioxide, and unconverted synthesis gas, and wherein that vapor mixture is processed by using a scrubbing solvent in a scrubbing column to separate both dimethyl ether and carbon dioxide from unconverted synthesis gas and wherein, subsequently, the diemethyl ether is separated from the carbon dioxide. The improvement comprises using, as the scrubbing solvent, a solvent comprising a mixture of dimethyl ether and methanol.
In a particular aspect of the invention, the effluent mixture is processed in a post-reactor flash column so as to produce the vapor comprising dimethyl ether, carbon dioxide, and unconverted synthesis gas, and said processing in the post-reactorflash column (xe2x80x9cpost-reactorxe2x80x9d indicating after the reactor) also produces a liquid comprising methanol, water, dissolved dimethyl ether and carbon dioxide.
In some embodiments of the invention, part or all of the liquid produced by the post-reactor flash column is recycled back to the dimethyl ether reactor. In such embodiments, or in other embodiments, the process of the invention comprises directing a part or all of the liquid produced by the flash column to a methanol dehydration reactor, and then additionally:
producing a dehydration reactor mixture comprising dimethyl ether, carbon dioxide, methanol and water;
directing said dehydration reactor mixture to a post-dehydration flash column;
producing a vapor and a liquid in said post-dehydration flash column;
directing the vapor produced in said post-dehydration flash column to a DME-CO2 column and therein separating DME of said vapor from CO2 of said vapor;
directing the liquid produced in said post-dehydration flash column to a water-methanol column and therein separating methanol of said liquid from water of said liquid; and
directing the methanol separated in the water-methanol column back to the methanol dehydration reactor.
In another aspect, related to the processing of liquid from the scrubbing column, the process of the invention further comprises:
directing the liquid stream from the scrubbing column to a post-scrubber flash column so as to produce a flash column vapor and a flash column liquid, said flash column vapor comprising dimethyl ether and carbon dioxide, said flash column liquid comprising a mixture of dimethyl ether and methanol,
directing said flash column liquid to a solvent recovery column so as to produce a recovery column vapor and a recovery column liquid, said recovery column vapor comprising dimethyl ether and carbon dioxide, said recovery column liquid comprising a mixture of dimethyl ether and methanol,
directing both the flash column vapor and the recovery column vapor to a DME-CO2 column capable of separating DME from CO2, and
directing the recovery column liquid to the scrubbing column so that said recovery column liquid becomes part of the scrubbing solvent in that column.
In a recycling aspect of the invention, the unconverted synthesis gas separated in the scrubbing column is recycled to the dimethyl ether reactor.
In another recycling aspect of the invention, the scrubbing solvent that exits the scrubbing column is recycled back to the scrubbing column, after being directed through one or more additional columns.
There are various preferred embodiments for carrying out the invention. In the scrubbing solvent, the molar fraction of dimethyl ether plus the molar fraction of methanol preferably equals at least 0.8, more preferably at least 0.95. Most preferably, the methanol and DME account for all of the scrubbing solvent except for trace elements (most typically less than one percent). In the scrubbing solvent, the ratio of dimethyl ether to methanol is preferably between 1/19 and 9, more preferably between 1/4 and 1, most preferably about 3/7. The effluent mixture is preferably cooled to 0 to 100xc2x0 F., more preferably, to 20 to 60xc2x0 F., as it exits the dimethyl ether reactor. The scrubbing solvent temperature in the scrubbing column is preferably between 0 and xe2x88x9260xc2x0 F., more preferably between xe2x88x9220 and xe2x88x9250xc2x0 F. The operating pressure of the scrubbing column is preferably between 300 and 1500 psig, more preferably between 400 and 900 psig. The operating pressure of the post-reactor flash column is preferably between 300 and 1500 psig, more preferably between 400 to 900 psig.
The liquid stream from the scrubbing column is preferably heated to a temperature between 50 and 250xc2x0 F., more preferably between 100 and 200xc2x0 F., prior to entering the post-scrubber flash column. The pressure of the post-scrubber flash column is preferably between 100 and 600 psig, more preferably between 300 and 500 psig. The pressure for the solvent recovery column range is preferably between 100 and 600 psig, more preferably between 300 and 400 psig. The pressure for the DME/CO2 distillation column is preferably between 100 and 400 psig, more preferably between 250 and 350 psig.